Radiosurgery is useful for treating tumors and other lesions by delivering a prescribed high dose of high-energy radiation to the target area while minimizing radiation exposure to the surrounding tissue. In radiosurgery, precisely focused beams of radiation (e.g. very intense x-ray beams) are delivered to a target region in order to destroy tumors or to treat the tumor for other purposes. The goal is to apply a lethal or other desired amount of radiation to one or more tumors, without damaging the surrounding healthy tissue.
Conventional radiosurgery uses a rigid and invasive stereotactic frame to immobilize the patient prior to diagnostic CT or MRI scanning. The treatment planning is then conducted from the diagnostic images. The treatment planning software determines the number, intensity, and direction of the radiosurgical beams that should be cross-fired at the target, in order to ensure that a sufficient dose is administered throughout the tumor so as to destroy it, without damaging adjacent healthy tissue. Immobilization of patient is necessary in order to maintain the spatial relationship between the target and the radiation source that ensures accurate dose delivery. The frame is fixed on the patient during the whole treatment process, causing pain and inconvenience to the patient.
Image-guided radiosurgery allows the elimination of such invasive frame fixation, during treatment. In an image-guided radiosurgical process, the patient position and the relative alignment of the radiation beam with respect to the patient target is continuously adjusted. In order to ensure the delivery of the correct dose of radiation to the correct location, the patient (and target) position during treatment needs to be detected. This is accomplished by registering the x-ray image acquired at the treatment time with the diagnostic 3D scan data (e.g., CT, MRI, ultrasound, or PET scan data) obtained pre-operatively at the time of treatment planning. In the field of medical image registration, this problem is categorized as a 2D/3D registration.
In the 2D/3D registration process, similarity measures are useful for comparing the image intensities in the x-ray images and the DRR images, so that the change in patient position (and thus in target region position) that has occurred between the diagnostic scanning and the taking of real-time images can be accurately detected. Image-guided radiosurgery requires precise and fast positioning of the target at the treatment time. In practice, the accuracy should be below 1 mm, and the computation time should be on the order of a few seconds. Unfortunately, it is difficult to meet both requirements simultaneously. In order to optimize the 2D/3D registration process in image-guided radiosurgery, it is necessary to provide an accurate, robust, and efficient similarity measure method and system.